channel: remove direct usage of ch_name

This is because the field can now be NULL and there were places where this
was not properly checked. So now a function call channel_get_name() should
be used instead and is garaunteed to return non-NULL.

In addition to this there is a nice added benefit. You can NOT set the name,
and it will automatically pull the name from the first available service. This
will not be persistent (part of config) unless you change that name and save
from the UI. This means if the service name changes, so will the channel name.

However on that last point there will not be, currently, any message sent to
the clients (HTSP) to inform them of such a change. So there might be some lag
in getting the update.